Heat-resistant high-strength stainless steel



June 20, 1967 Original Filed Oct. 5,

sTREss (I000 ps1) RUPTURE TIME (Hours) A. KASAK ETAL Re. 26,225HEAT-RESISTANT HIGH-STRENGTH STAINLESS STEEL 1961 4 Sheets-Sheet 1TENSILE STRENGTH AT FiOOM TEMPERATURE o 25% FREE FERRITE AT2000F 250- oYIELD STRENGTH AT ROOM TEMPERATURE ,A OGEISILE STRENGTH AT "00F 3 YIELDSTRENGTH AT ||O0F X loox g b. 50 v so AT 40 FigJ TEMPERING AT |2oo|= 0 lI I I I l I I I I o 2 4 s e PERCENT MOLYBDEN UM ||00F 75000 psl Fig.2200- o O Q 5 \LIZOO F 3 OOOPSI G I I I I I I I 0 2 4 s a PERCENTMOLYBDENUM INVENTORS August Kusok BY Edward J.Dulis VIJA! K. ChundhokHEAT-RESISTANT HIGH-STRENGTH STAINLESS STEEL 4 Sheets-Sheet OriginalFiled Dot. 5, 1961 Fig.5

I I 0.6 0.8 L0 PERCENT VANADIUM m mu w w 2 3 5 E3 MEDEDm STEEL 77 S L EE T S H w T E G E N T M S K T S A H W S m T H H O E l H T C S C W. m E mU m M E S m m m m. 5 MW 5 O 5 2 2 I @380; 15255 wmnEnmiwwmu E6169 Fig.6

United States Patent tion of New Jersey Original No. 3,154,412, datedOct. 27, 1964, Ser. No. 143,157, Oct. 5, 1961. Application for reissueOct. 26, 1966, Ser. No. 596,035

9 Claims. (Cl. 75-126) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

This invention pertains to stainless steel and particularly to stainlesssteel exhibiting very high strength at room and elevated temperatures.More particularly, the invention pertains to stainless steel having veryhigh strength and exhibiting good oxidation and corrosion resistanceunder substantially atmospheric conditions at both ambient and elevatedtemperatures.

Concerning the various steels of the prior art, those known asaustenitic stainless steels may be characterized as resistant tocorrosion and oxidation and as having good strength at elevatedtemperatures. A disadvantage of such steels, however, is theirrelatively low strength at ambient and moderately elevated temperatures.On the other hand, some steels of a heat-treatable nature and having amartensitic structure attain desired high strengths at ambienttemperature which are retained at elevated temperatures. Exemplary ofsuch steels are those covered by US. Patent No. 2,986,463, issued May30, 1961, which was copending herewith. These steels, however are notcharacterized by good resistance to corrosion and oxidation. Othermartensitic steels having chromium contents of about 12% by weight haveimproved corrosion and oxidation resistance while similarly exhibitingrelatively good strength at low temperatures and fairly high strength atelevated temperatures. The latter martensitic steels however, arelimited in chromium content to a maximum value of about 12%. Above thisvalue, the microstructure of the steel is characterized by the presenceof substantial amounts of free ferrite. Free ferrite in substantialamounts is undesirable, being generally regarded as the probable causeof many hot-working difficulties as well as excessive variations inproduct properties with regard to the direction of working.

Another group of heat-treatable stainless steels, the socalledsemiaustenitic steels, have chromium contents up to about 17%, but theirapplicability at temperatures above about 900 to 1000 F. is verylimited. At such temperatures, particularly after prolonged exposure,these steels have relatively low strength values.

In accordance with a principal object of the present invention, steelsare provided which are heat-treatable to very high strength levels andwhich retain an unusually high percentage of such strength attemperatures up to 1200 F. or higher. These steels offer particularlyhigh strengths, in both showtime and prolonged exposures, attemperatures ranging from about 1000 to 1200 F. and in addition exhibithigh resistance to corrosion and oxidation. The inventive steelsconstitute a substantial improvement over the elevated-temperaturestructural materials known to the art and are particularly well suitedfor construction of aircraft and engine parts subjected to widevariations in operating temperatures.

The steels of the invention comprise iron-base alloys of the followingbroad composition ranges:

Percent Carbon 0.01 to 0.25 Nitrogen 0.0l to 0.25 Sum of carbon andnitrogen 0.06 to 0.35 Manganese Up to 1.0 Silicon Up to 1.0 Chromium 11to 16 Vanadium Up to 1.0 Molybdenum Up to 10 Tungsten Up to 10 Sum ofmolybdenum and tungsten 3.5to 20 Cobalt 10 to 20 Aluminum Up to 0.25Boron Up to 0.025

Preferred composition ranges of the inventive steels are as follows:

Carbon 0.05 to 0.17% 0.00 to 017 Nitrogen 0.0lto0.07f 13 Sum of Carbonand Nitrogen 0.10 to 0.20%

Manganese. 0.10 to 0.40%.

Silicone-.- 0 10 to 0.40"

In addition to the elements listed in the above broad and preferredcomposition ranges, the steels may contain impurities commonlyencountered in the steelmaking practice (such as sulfur and phosphorus)and tramp elements. i

The steels of this invention are typified by Steels 77 and 107, thechemical compositions of which, together with those of a sizeable numberof steels of similar compositions are given in Tables LA and I-B. TableI-A lists these steels according to significant compositional variable,whereas Table I-B lists the same steels according to identificationnumber. Microstructural characteristics of the steels are determined bymicrostructural examination and mechanical properties of the steels asdetermined by hardness, tension, and creep-rupture tests are set forthin Tables Il through VI.

TABL

Results of Tension Tests at 1100 R 5 Austenitiz 0.2%Oi1set TensileElong. in Reduction Steel ing Tem- Yield Strength 1 inch of Area.

perature Strength (1,000 psi.) (Percent) (Percent) quenched,refrigerated at, 100 F. 2+2 hr.

TABLE V Results of Creep-Rupture Tests at 1100 F.

Rupture Aust. Stress Steel Temp. (p.s.i.)

( F1) Life Elong. Red. of

thr.) (percent) Area (percent) 2, 000 75, 000 112 8 11 l' 1, 900 75, 000239 8 10 t 2, 000 2 15, 000 365 5 9 2, 000 75, 000 224 17 51 2, 100 15,000 195 13 35 1, 900 75, 000 221 13 31 1 1,900 00, 000 137 21 47 77 2,000 15, 000 366i 8 14 2,000 00, 000 104 13 15 2, 100 I5, 000 406 4 7 2,000 75, 000 243 5 8 2, 100 75, 000 259 3 8 J 2, 000 75, one use 9 9 i 2,000 so, 000 31 I8 11 11 1, 900 75, 000 296 13 16 2, 000 75, 000 309 7 122, 100 75, 000 275 5 11 2, 000 000 214 12 23 2, 000 75, 000 269 11 21 2,000 75, 000 224 9 22 2, 000 75, 000 259 8 15 2, non 75,000 242 7 13 f 1,900 75. D00 1119 12 30 l 2, 000 I5. 000 254 11 20 2, 000 15, 000 159 1127 2, 000 T5, 000 274 13 26 2, 000 75, 000 7 19 55 2, 000 75, 000 0, 121 01 2,000 75,000 115 21 49 2, D00 75. 000 212 13 30 2, 000 75, 000271i 5 8 1, 900 75, 000 185 20 55 2, 000 75. 000 694 B 14 2, 000 75, 000257 11 PR 2, 000 75, 000 338 11 17 2, 000 75, 000 782 5 7 1 Heattreatment: Austenitired at the indicated temperature, oilquenehed,reirigerated at 100 F. 16 hr,, and tempered at 1,100 F. for 2 2hr.

I; Heat treatment: Austenitized at the indicated temperature, oilquenched, refrigerated at 100 F. 15 hr., and tempered at 1,100 F. for

6 TABLE VI Results of Creep-Rupture Tests at 1200 F. Under 21 Stress or35,000 p.s.i.

Rupture Steel Aust.

Temp. F. Elong. Red. 01

Lite (hlx) (percent) Area.

(percent) 2, 000 9|] 13 13 2, 000 148 16 26 2, 000 103 18 17 2, 000 7616 24 2, 000 114 11 11 1, 900 63 32 50 2, 000 135 21 28 2, 100 109 25 332, 000 as 27 27 2, 000 63 17 22 2, 000 66 51 47 2, 000 16 19 2, 000 69 99 l, 900 21 29 54 2, 000 73 32 43 2, 000 84 7 12 2, 000 56 21 39 2, 00027 16 21 2, 000 6 19 31 2, 000 36 8 19 2, 000 72 43 38 2, 000 103 11 151, 900 54 42 53 2, 000 98 13 28 2, 000 146 16 1 7 2, 000 70 22 24 2, 00082 27 38 2, 000 101 25 33 2, 000 175 11 13 lleat treatment: Austenitizedat the indicated temperature, oil quenched, refrigerated at F. )6 hr.,and tempered nt U F. for 2+2 hr.

Graphical illustrations of the foregoing, as well as graphicalcomparisons of the steel of the invention with prior-art steels, aredepicted by the drawings, wherein;

FIG. 1 shows the effect of molybdenum on tensile properties andhardness;

FIG. 2 shows the efi'ect of molybdenum on creep-rupture strength;

FIG. 3 shows the eiTect of cobalt on tensile properties, hardness andmicrostructure;

FIG. 4 shows the eifect of cobalt on creep-rupture strength;

FIG. 5 shows the effect of vanadium on creep-rupture strength;

FIG. 6 shows the creep-rupture strength of various prior-art steelstogether with that of a steel typical of the invention;

FIG. 7 shows the tensile strength of various prior-art steels togetherwith that of a steel typical of the invention;

FIG. 8 shows the oxidation resistance of various priorart steelstogether with that of a steel typical of the invention.

As indicated by the test data set out in the aforementioned tables anddrawings, steels within the invention are capable of exhibiting hardnessof Rockwell C 48 or greater and tensile strength of 250,000 p.s.i. orgreater at room temperature, tensile strength of 170,000 p.s.i. orgreater at a temperature of 1100 F., creep-rupture life of 100 hours ormore for an applied stress of 75,000 p.s.i. at a tempcrturc of 1100 F.,and creep-rupture life of 50 hours or more for an applied stress of35,000 psi. at a temperature of 1200" F. In addition to exhibitingsuperior strength properties the steels of the invention exhibit goodductility at both room and elevated temperatures and have excellentweldability.

In accordance with the basic concepts of the present invention, theforegoing superior mechanical properties are obtained only upon theachievement of a proper compositional balance and the application of aproper heat treatment. Such heat treatment may involve the steps ofaustenitizing at an elevated temperature, preferably in the range of1900 to 2100 F., cooling to room temperature, and heating at an elevatedtemperature below about 1500 F. Refrigeration at a subzero temperature,e.g. l F., is preferably included in the heat-treating scheduleimmediately following the austenitizing step for the purpose offacilitating the transformation of austenite. While the recited sequenceof heattreating steps has been found admirably suited for thecontemplated purpose, it is well to note that other sequences ofheattreating steps, including conditioning treatments, may be employed,the principal criteria being the complete or substantially completeelimination of free ferrite at elevated temperatures and the complete orsubstantially complete elimination of austenite thereafter as well asthe obtention of high strength.

It is also understood that a combination of thermal and mechanicaltreatments may result in further improvement of some of the importantproperties of the inventive steels. For example, when a sheet sample ofSteel 77 was cold reduced 50% in thickness and subsequently heated at1100" F. for two plus two hours, a tensile strength of 341,000 psi. andan elongation of 6% were obtained at room temperature. In contrast, itwill be noted from Table III that thermal treatment alone of Steel 77resulted in a maximum tensile strength of only 294,000 p.s.i.

It is well known to the art that in order for a steel to exhibitsatisfactory corrosion resistance at ambient temperature andsatisfactory oxidation resistance at temperatures up to about 1200 F.,the chromium content of such steel should be about 12% or more. Ingeneral, the higher the chromium content of the steel, the higher itscorrosion and oxidation resistance. In view of these considerations,chromium has been included as an essential element in the steels of theinvention, the minimum content thereof being set at 11%. As it isdesirable to maintain the chromium content at the highest levelcompatible with desired microstructural characteristics, i.e., essentialfreedom from free ferrite and retained austenite, the maximum chromiumcontent of the inventive steels has been set at about 16%. The latterfigure finds support in the accompanying data wherein it is observedfrom Tables I-A and II that when the compositional variable is chromium,values of 14.36% (Steel 77) and 15.54% (Steel 97) result in amicrostructure having no free ferrite after an austenitizing treatmentat 1900 F., whereas, a value of 16.20% (Steel 98) results in amicrostructure having 10% free ferrite after the same austenitizingtreatment.

Molybdenum has been included in the steels of the invention as aneffective strengthener. Supporting data therefor were obtained bytesting steels of the basic inventive composition wherein the molybdenumcontent was varied (see Table IA, Steels 102, 103, 77 and 104). As shownin FIGS. 1 and 2, all the important strength properties, i.e., yield andtensile strengths at room temperature and at 1100 F., hardness aftertempering at 1100 F. and 1200 F., and rupture times in creep-rupturetests at 1100 F. and 1200 F., are steadily enhanced as the molybdenumcontent is increased from about 0.1 to about 5%. Tables III through VIindicate that for this same molybdenum range, ductility is maintained ata high level. Molybdenum contents in excess of about 5% have someadditional strengthening effects in short-time tests; however, thetendency for formation of free ferrite at high temperatures isincreased. Therefore, although molybdenum contents up to about areincluded within the broad scope of the invention, a molybdenum contentof about 4 to 6% is preferred for an optimum combination of properties.In addition to its strengthening role, molybdenum functions to increasesignificantly the corrosion resistance of the inventive steels in suchmedia as chloride solutions.

Tungsten is also an effective elevated-temperature strengthener in theinventive steels. For example, a steel containing 5.11% tungsten andonly 0.04% molybdenum 8 (Steel 122) exhibited a higher creep-rupturelife (782 hours) than any other stecl when tested at 1100" F. under astress of 75,000 psi. (see Table V). Similarly, when tested at 1200 F.under a stress of 35,000 p.s.i., this same steel again exhibited ahigher creeprupture life (175 hours) than any other steel tested (seeTable VI).

In addition to the benefits derived from the individual uses of tungstenand molybdenum, test data show that a combination of these elements isan effective strengthening agent in the inventive steels. In supportthereof, it will be noted that Steel 75, containing 3.01% molybdenum and2.35% tungsten, exhibited a creep-rupture life of 365 hours when testedat 1100 F. under a stress of 75,000 psi. (see Table V).

Cobalt has been incorporated as an essential ingredient in the inventivesteels for the purpose of obtaining desired microstructuralcharacteristics, a critical cobalt range of 10 to 20% having beendetermined therefor. In this regard, Table II strikingly reveals thatcobalt values below about 10% (Steels 111, 112 and 113) result in theformation of excessive amounts of free ferrite, and that cobalt valuesabove about 10% (Steels 76, 106, 77, 78 and 67) permit the complete orsubstantial elimination of free ferrite by proper compositional balanceand/or heat-treatment selection. On the other hand, cobalt contents inexcess of about 15% permit the retention of increasing amounts ofaustenite, the effect of which is to lower the room-temperature strengthproperties of the steels to some extent. This is graphically depicted inFIGS. 3 and 4, wherein it is observed that optimum strength values atboth room temperature and 1100 F. are obtained in a cobalt range ofabout 13 to 15% and that said values fall oflf with increasing cobaltcontent until an extrapolated maximum of about 20% cobalt is reached,beyond which value strength properties become unsatisfactory.

In addition to the employment of a critical cobalt range, the inventioncontemplates as a preferred embodiment a proper balancing of the cobaltcontent within said range with the nitrogen content. This properbalancing, as evidenced by Steels 76, 106, 77, 78 and 67, involves therestriction of the cobalt content to values in excess of 12% where thesteel is essentially nitrogen-free or low in nitrogen, i.e., less thanabout 0.1%. Where the nitrogen present amounts to about 0.10% orgreater, the cobalt content may be lowered to the range 10 to 12%. Itwill be noted from Table II after an austenitizing treatment at 1900 F.that Steel 76, embraced within the broad concept of the invention,exhibits 5% free ferrite and that Steels 106, 77, 78 and 67, comprisingpreferred embodiments of the invention, exhibit not more than 1% freeferrite after the same austenitizing treatment.

As is known to the art, the alloying elements cobalt and nickel areinterchangeable to some extent for the formation of an austeniticstructure and for the elimination of free ferrite in steel. Asincreasing amounts of nickel up to 10.14% (Steel 118, and 114) were usedin the steels of this invention as a replacement for cobalt, it becameevident that While nickel effectively caused the amount of free ferriteto decrease, it also stabilized the austenitic structure of the steel sothat when the free ferrite was fully eliminated (as in Steel 114containing 10.14% nickel), the steel was not hardenable and thusincapable of exhibiting very high strengths at ambient and moderatelyelevated temperatures. Therefore, it is recognized that although to someextent nickel may be substituted for cobalt in these steels, no completeor substantial replacement of cobalt with nickel is possible.

The elements carbon and nitrogen both constitute strong austeniteformers and are essential in the steels of this invention for theelimination or minimization of free ferrite in the microstructure.Quantitatively, it appears that about 0.06 to 0.35% carbon plus nitrogen(combined) is required to avoid substantially the formation of freeferrite upon heat treatment at high temperatures. However, it isimportant to select quantities Within the foregoing range so as tomaintain a proper compositional balance vis-a-vis ferrite formers, lestthe ferritizing effect of the latter become dominant. For example, Wherethe sum of carbon plus nitrogen is less than about 0.1% the presence ofvanadium (a strong ferrite former) cannot be tolerated above about 0.1%.In this regard it will be noted that Steel 121 (containing 0.068% carbonplus nitrogen and no intentionally added vanadium) was heattreatable tozero precent free ferrite, while Steels 11S and 116 (containing 0.076%

of molybdenum because of the substantial difference in the atomicweights of these two elements. In the steels of this invention weconsider tungsten as at least a partial replacement for molybdenum.

For the purpose of comparing relative strength and corrosion properties,an investigation was undertaken along these lines with respect to thesteels of the invention and prior-art steels of the semiaustenitic, the12% chromium high-strength, and the chromium hot-work tool-steel types.The nominal compositions of representative examples of carbon plusnitrogen and 0.039% carbon plus nitrogen, respectively as well as aboutthe latter steel types are set forth in Table VII.

Table VII Nominal Compositions of Comparison Steels Designation t C l MnI S1 Cr N1 V Mo W Al N F0 Semiaustenitle Steels:

AM 350 0.10 1.00 0. 40 1e 5 PH-7M0 0. 09 1. 0 1. 0 14. 0-15. 0

HIQX. IIIELX 1118K.

12% Cr High-Strength Steels:

Crucible 422 1 0 22 0. 55 0. 35 12.0

Crucible 422M 0. 2s 0. s4 0. 25 12. 0

Lapelloy 4 0. 1. 0 0. 25 12. 0 Hot-Work Steels:

H-ll 3 0. 38-0 43 0. 20-040 0. 50-1. 00 4. 75-5. 25 0. -0. 50 1.20-1.40m1.

Crucible 21s 0. 35 0.35 1.00 5.00 0. 45 1. Bel.

1 Grade of Allegheny-Ludluni Steel Co.

2 Grade of Armoo Steel Co.

3 AISI Type.

4 Grade of General Electric Co.

6 Grade of Crucible Steel Co. of America.

one-half percent each of vanadium) were heat-treatable to no less than20% and 30% free ferrite, respectively. On the other hand, it has beenfound that high carbon plus nitrogen contents have an adverse effect onductility at high strength levels and promote the retention of austenitein the microstructure. Therefore, to ensure proper compositionalbalance, it is preferred in the steels of this invention to keep thecarbon plus nitrogen content between about 0.10% and 0.20% when thecontent of cobalt (as indicated heretofore, also a strong austeniteformer) is about 12% or higher, and between about 0.20% to 0.30% whenthe content of cobalt is below about 12%.

Vanadium in amounts up to about 1.0% may be added to the steels of thisinvention to improve certain properties. As shown in FIG. 5, additionsof vanadium to the steels of the invention increased rupture life at1100 F., as determined by creep-rupture tests, the highest rupture lifebeing obtained with a steel containing about 0.5% vanadium.

It is generally agreed that tungsten has alloying effects Comparativedata relating to creep-rupture strength and elevated-temperature tensilestrength for the steels in Table VII, representative of the typesrecited therein, and a sample of Steel 77, representative of the steelsof the invention, are given in Tables VIII and IX, respectively.

Semiaustenitie Steels:

PHlb-H'lo 12% Cr High-Strength Steels:

Crucible 422 Crucible 42201..."

Lapelloy Ho t-Work Steel: ru-

. bl 21s 155 100 in steels quite slmllar to those of molybdenum except si s S Invumion; 40 that the amounts (in weight percent) necessary toproduce S1081 77 169 35 the effects with tungsten are about twice asgreat as those TABLE IX Comparison of Elevated-Temperature TensileStrengths Steel Condition R.T. 200 300 400 500 500 700 800 000 1,0001,100 1,200

F. F. F. F. F. F. F. F. F. F. F.

Semiaustenitic Steels:

11111-355 SCT l 222 208 20s 20s 197 180 200 185 101 170 179 155 105 0845 5-7Mo 238 227 211 203 182 101 12% Chromium High-Strength Steels:

Crucible 422 57 Crucible 422M T4 Hot-Work Steel: H-ll 100 Steal of theInvention: Steel 77 121 l Subzero-cooled and tempered. 2 Roll-hardenedat room temperature and aged 950 F. B quenched and tempered.

Graphical depiction of the foregoing data is shown in FIGS. 6 and 7,wherein the superiority of the steels of the invention over thecomparison steels of the prior art is clearly evident.

To assess the corrosion resistance of the steels of this invention, aheat-treated sample of Steel 77 was subjected to the water-vapor-columncorrosion test and to the CASS test along with heat-treated samples of anumber of the aforementioned prior-are comparison steels. These testshave been used for many years as laboratory tests to indicate therelative corrosion resistance of stainless steels under essentiallyatmospheric conditions.

In the water-vapor-column test, the samples are exposed to water vaporcondensation for a period of 8 hours and then allowed to dry for aperiod of 4 hours. This procedure is repeated several times. Results ofthis test after six wet-and-dry cycles showed the H-ll specimen to becovered with heavy rust, and the Crucible 422 steel to contain localrust spots, whereas it showed Steel 77 of this invention to be onlyslightly stained, as were the samples of AM 350 and PH157M0.

In the so-called CASS test, the specimens are sprayed with an acidifiedaqueous solution of NaCl and CuCl for 16 hours at 120 F. The samples arethen cleaned with a detergent and the surface appearance is rated byvisual examination. In this test the H-ll sample was severely attackedand Crucible 422 was seriously pitted; Steel 77 along with otherstainless steels showed only slight pitting and all to about the samedegree.

To determine the oxidation resistance, samples of Steel 77 were exposedto a static air atmosphere at 1200 F. and measurements of weight gainwere made periodically; in this test, a higher rate of weight gainindicates a higher rate of deterioration (scaling) of the steel in airat the test temperature. The results of this test, given in FIG. 8, showclearly that Steel 77 of this invention exhibited a superior resistanceto oxidation attack.

For the purpose of evaluating Welding characteristics of steels of theinvention, welding tests were conducted on sheet samples of Steel 77using the semiautomatic inert-gas-shielded-tungsten-arc process. Thetests involved tension testing of one-eighth inch thick specimens, bothwelded and unwelded. The average results of these tests are given inTable X.

These results indicate a weld efliciency (i.e., ratio of tensilestrengths of the welded specimens to those of the unwelded specimens) of98%. Also, it is important to note that no high-temperature heattreatment is needed after the welding operation. This feature isextremely important in fabrication, since it virtually precludes anyditliculties resulting from scaling and warping of a formed and weldedarticle.

While the invention has been described and disclosed in connection withspecific embodiments thereof, it will be understood that the inventionis not limited thereto but may be otherwise embodied or practiced withinthe scope of the appended claims.

We claim:

1. A stainless steel havnig enhanced strength and useful ductility overa temperature range from subzero to about 1200 F., consistingessentially, by weight percent, of:

Element: Percent Carbon+nitrogen 0.06 to 0.35. Chromium From 11 to 16.Molybdenum-l- /z tungsten From 3 to less than 7. Nickel Up to 3. VandiumUp to 1. Nickel+cobalt 10 to 20. Manganese Up to 1. Silicon Up to 1.Aluminum Up to 0.25. Boron Up to 0.025. Iron Balance.

wherein vanadium is present on the low side of its range when the sum ofcarbon+nitrogen is on the [high] low side of the combined range of thoseelements.

2. A stainless steel having enhanced strength and usewhere the sum ofcarbons-nitrogen is in the range of 0.10 to 0.20 when the sum ofnickel-l-cobalt is at Least 12% and is in the range of 0.20 to 0.30 whenthe sum of nickel-l-cobalt is less than 12%.

3. A stainless steel having enhanced strength and useful ductility overa temperature range from subzero to about 1200 F., consistingessentially, by weight percent, of:

Element: Percent Carbon 0.01 to 0.25. Nitrogen Less than 0.15.Carbon+nitrogen 0.06 to 0.35. Chromium 12 to 15.5. Molybdenum-P/ztungsten 4to 6. Cobalt 12 to 14. Vanadium Up to 0.5. Manganese Up to0.4. Silicon Up to 0.4. Aluminum Up to 0.25. Boron Up to 0.025. IronBalance.

4. A heat-treated stainless steel consisting essentially of thecomposition of claim 1, said steel being characterized by the followingminimum mechanical properties:

Hardness Ro-ckwell C 48 Tensile strength (room temperature) p.s.i250,000 Tensile strength (1100 F.) p.s.i 170,000

5. A heat-treated stainless steel consisting essentially of thecomposition of claim 2, said steel being characterized by the followingminimum mechanical properties:

Hardness Rockwell C 48 Tensile strength (room temperature) p.s.i 250,000Tensile strength (1100 F.) p.s.i 170,000

6. A heat-treated stainless steel consisting essentially of thecomposition of claim 3, said steel being characterized by the followingminimum mechanical properties:

Hardness Rockwcll C 48 Tensile strength (room temperature) p.s.i 250,000Tensile strength (1100 F.) p.s.i 170,000

Hrs. Creep-rupture life (75,000 p.s.i. at 1100 F.) 100 Creep-rupturelife (35,000 p.s.i. at 1200 F.) 50

8. A heat-treated stainless steel as in claim 5, said steel beingadditionally characterized by the following minimum mechanicalproperties:

Hrs. Creep-rupture life (75,000 psi. at 1100 F.) 100 Creep-rupture life(35,000 p.s.i. at 1200 F.) 50

9. A heat-treated stainless steel as in claim 6, said steel beingadditionally characterized by the following minimum mechanicalproperties:

Hrs. Creep-rupture life (75,000 p.s.i. at 1100 F.) 100 Creep-rupturelife (35,000 p.s.i. at 1200 F.)

References Cited The following references. cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 1,357,549 11/1920 Fahrenwald 128 2,432,614 12/1947Franks 75126 2,462,665 2/1949 Olcott 75126 2,848,323 8/1958 Harris etal. 75-126 2,880,085 3/1959 Kirkby et al. 75-126 2,990,275 6/1961 Binderet al 75-126 DAVID L. RECK, Primary Examiner. P. WEINSTEIN, AssistantExaminer.

